I would change it just to be safe. There is a vent on the differential housing. If you are interested there is a post under the Mod section on how to install remote vent with a hose and a couple of fittings inside the fuel door.
Gear oil is cheaper than a differential! Go with Synthetic Oil.

No. It's just a bellows that expands and contracts to accommodate air displaced from the diff. Remember the diff doesn't need to "breathe" per se. The air inside the diff simply needs a place to go when the diff fluid expands/contracts with temperature. Click on the picture for more info.

No. It's just a bellows that expands and contracts to accommodate air displaced from the diff. Remember the diff doesn't need to "breathe" per se. The air inside the diff simply needs a place to go when the diff fluid expands/contracts with temperature.

Click to expand...

That's cool , kinda like an expansion tank concept . So essentially that makes it a closed system ?